The present invention relates to semiconductor optical devices and a method of manufacturing them and, more particularly, to a semiconductor optical device having two sides of an active region buried in a semi-insulating crystal and a method of manufacturing the device.
A semi-insulating buried heterostructure (SIBH) having a semi-insulating layer as a buried layer is used for a semiconductor optical device such as a semiconductor laser diode or semiconductor optical modulator. It is known that when this structure is used for such a device, lower device capacitance and higher speed modulation can be realized than when a p-n buried structure is used. For this reason, a semi-insulating buried heterostructure is indispensable to semiconductor optical modulators and semiconductor optical devices used for a high capacity optical transmission system.
Conventionally, an iron (Fe)-doped semiconductor crystal is generally used for such a semi-insulating buried layer. Of the doped Fe, electrically activated Fe acts as a compensator for supplied from an n-type dopant in the film, thereby forming a high-resistance film.
A problem in this technique is that interdiffusion between iron (Fe) as a dopant in the semi-insulating buried layer and zinc (Zn) as a dopant in a p-cladding layer and p-contact layer of the device occurs at an interface between the Zn-doped layer and the buried layer. As a consequence, zinc is diffused into the semi-insulating buried layer, resulting in a deterioration in the characteristics of the device. This has been a factor that causes a deterioration in the modulation characteristics of the device, in particular. This phenomenon is not limited to Zn but equally applies to other p-impurities (Be, Cd, Mg, and the like).
For this reason, conventionally, the Fe doping concentration into a semi-insulating buried layer (the Fe atom concentration in the film) is limited to be equal to or lower than a concentration at which interdiffusion becomes noticeable. In this case, a sufficiently high resistance cannot be attained.
As disclosed in Japanese Patent Laid-Open No. 6-275911, when a buried layer is doped with Fe in epitaxial growth, even if the same amount of source gas containing Fe is supplied, the doping concentration varies depending on the crystallographic orientation of growth surface as shown in FIG. 5.
FIG. 5 shows the saturation concentrations of Fe electrically activated in semiconductor crystal in various crystallographic orientations (characteristic curve c), the Fe content (doping concentration) when dicyclopentadienyliron (Cp2Fe) is supplied at 10 sccm (characteristic Curve a), and the impurity concentrations at the time of undoped growth (characteristic curve b). The abscissa represents the off angle from the (100) orientation to the [01-1] orientation, with the main crystallographic orientations being indicated by the arrows; and the ordinate, the concentration in cm−3.
As is obvious from the characteristic curve a in FIG. 5, the concentration gradually decreases from the (011) facet to the (100) facet at first and reaches its minimum value near the (111)B facet. Thereafter, the concentration gradually increases. That is, when dicyclopentadienyliron (ferrocene, Cp2Fe) is supplied at 10 sccm, the Fe content decreases from the (011) facet to a position near the (111)B facet, and then increases afterward to reach its maximum value near the (311)B facet. Thereafter, the content decreases toward the (100) facet.
As indicated by the characteristic curve b in FIG. 5, the impurity concentration at the time of undoped growth almost linearly increases from the (011) facet to a position near the (111)B facet. Thereafter, the concentration abruptly decreases to become a very small value near the (211)B facet and (311)B facet, and then gradually increases toward the (100) facet.
As is obvious from the characteristic curve a, when a (111)B facet is formed in the burying growth process, the concentration of electrically activated Fe becomes insufficient, and hence a high-resistance crystal cannot be obtained.
As shown in FIG. 6, when a mesa stripe is to be buried, a facet having a crystallographic orientation different from the crystallographic orientation of the substrate is formed near the mesa stripe in the growth process (Japanese Patent Laid-Open No. 6-275911).
FIG. 6 shows changes in a growth surface in the burying growth process. As is understood from FIG. 6, a mesa stripe 10a extends in the [110] direction.
When this mesa stripe 10a is to be buried by the metalorganic vapor phase epitaxy method (MOVPE method), a major growth surface on a side wall of the mesa stripe 10a extends from the (011) facet to the (100) facet through the (111)B facet.
In the burying growth process accompanying such changes in growth surface, if a layer is grown while the flow rate of dicyclopentadienyliron as an Fe source is kept constant, the Fe content considerably decreases after a (111)B facet is formed as compared with that before the (111)B facet is formed. At the same time, the concentration of an impurity other than Fe, which interferes with an increase in resistance, increases. For this reason, the resistivity of a portion grown after the formation of the (111)B facet decreases. As a consequence, a layer having a sufficiently high resistance cannot be formed.
If the Fe doping concentration is increased to improve this, the Fe doping concentration in a portion grown before the formation of the (111)B facet increases. This enhances interdiffusion.
Under the circumstances, the Fe doping concentration has its own upper limit. That is, a buried layer is doped with Fe in growth to a (100) facet up to a concentration at which a high resistance can be obtained, but a sufficiently high resistance cannot be obtained in growth to a (111)B facet exhibiting lower doping efficiency than the (100) facet.
As described above, in the conventional technique, a layer with a sufficiently high resistance cannot be obtained.
Recently, it has been found that in a semi-insulating semiconductor crystal doped with ruthenium (symbol of element: Ru), almost no interdiffusion occurs between Ru and Zn, and the manufacture of a semiconductor laser using Ru-doped semi-insulating buried layers has been reported (“A. Dadger et.al, Applied Physics Letters Vol. 73, No. 26 pp. 3878-3880 (1998)”, “A. Van Geelen et.al, 11th International Conference on Indium Phosphide and Related materials TuB 1-2 (1999)”).
However, no study has been made on the relationship between the Ru doping concentration and the crystallographic orientation or device characteristics.
In order to obtain satisfactory device characteristics, it is essential to form an optimal semi-insulating buried layer.
For this purpose, for example, a flat buried layer region grown on a (100) facet in a crystallographic orientation of a substrate must be increased in resistance, and the resistance of a buried layer near a side of a buried mesa stripe of the device must be sufficiently increased.
In the burying growth process on a side of a mesa stripe, however, a growth mode occurs in a crystallographic orientation (typically the orientation of (111)B) different from the crystallographic orientation of grown on the (100) facet. For this reason, in the growth method using an Ru doping condition that increases the resistance of only a semiconductor crystal grown on the (100) facet, the resistance of a buried layer on a side of a mesa stripe cannot be sufficiently increased, and hence sufficient device characteristics cannot be obtained.
Demands therefore have arisen for a growth method using an Ru doping condition for a semi-insulating buried layer, under which sufficient device characteristics can be obtained.